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The fluid mechanics of root canal irrigation
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2010 Physiol. Meas. 31 R49
(http://iopscience.iop.org/0967-3334/31/12/R01)
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IOP PUBLISHING
PHYSIOLOGICAL MEASUREMENT
Physiol. Meas. 31 (2010) R49–R84
doi:10.1088/0967-3334/31/12/R01
TOPICAL REVIEW
The fluid mechanics of root canal irrigation
K Gulabivala1,4 , Y-L Ng1 , M Gilbertson2 and I Eames3
1 UCL Eastman Dental Institute, University College London, 256, Grays Inn Road,
London WC1X 8LD, UK
2 Department of Mechanical Engineering, University of Bristol, University Walk, Bristol,
BS8 1TR, UK
3 Department of Mechanical Engineering, University College London, Torrington Place,
London WC1E 7JE, UK
E-mail: [email protected]
Received 17 June 2009, accepted for publication 15 October 2010
Published 12 November 2010
Online at stacks.iop.org/PM/31/R49
Abstract
Root canal treatment is a common dental operation aimed at removing the
contents of the geometrically complex canal chambers within teeth; its purpose
is to remove diseased or infected tissue. The complex chamber is first enlarged
and shaped by instruments to a size sufficient to deliver antibacterial fluids.
These irrigants help to dissolve dying tissue, disinfect the canal walls and
space and flush out debris. The effectiveness of the procedure is limited
by access to the canal terminus. Endodontic research is focused on finding
the instruments and clinical procedures that might improve success rates by
more effectively reaching the apical anatomy. The individual factors affecting
treatment outcome have not been unequivocally deciphered, partly because of
the difficulty in isolating them and in making the link between simplified,
general experimental models and the complex biological objects that are
teeth. Explicitly considering the physical processes within the root canal can
contribute to the resolution of these problems. The central problem is one of
fluid motion in a confined geometry, which makes the dispersion and mixing
of irrigant more difficult because of the absence of turbulence over much of
the canal volume. The effects of treatments can be understood through the
use of scale models, mathematical modelling and numerical computations. A
particular concern in treatment is that caustic irrigant may penetrate beyond
the root canal, causing chemical damage to the jawbone. In fact, a stagnation
plane exists beyond the needle tip, which the irrigant cannot penetrate. The
goal is therefore to shift the stagnation plane apically to be coincident with
the canal terminus without extending beyond it. Needle design may solve
some of the problems but the best design for irrigant penetration conflicts with
that for optimal removal of the bacterial biofilm from the canal wall. Both
irrigant penetration and biofilm removal may be improved through canal fluid
4
Author to whom any correspondence should be addressed.
0967-3334/10/120049+36$30.00
© 2010 Institute of Physics and Engineering in Medicine
Printed in the UK
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Topical Review
agitation using a closely fitting instrument or by sonic or ultrasonic activation.
This review highlights a way forward by understanding the physical processes
involved through physical models, mathematical modelling and numerical
computations.
Keywords: irrigation, root canal, endodontics, fluid dynamics, viscous flows
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Periapical disease is an inflammatory response around root canal termini to intra-radicular
bacterial infection. In 1992, over £43 million were spent on root canal treatment in the
general dental services of the National Health Service in the UK (Dental Practice Board
2005). This consistent trend, which does not take account of an increasing amount of care
provided under private contract by specialist endodontists, is mirrored in the USA (Wayman
et al 1994, Boykin et al 2003) and even in the low caries rate adult Danish population
(Bjørndal and Reit 2005). The problem has posed a substantial economic and manpower
burden to modern society (Figdor 2002) and is a frequent cause of work absence and cost to
industry. The reported success rates of root canal treatment, defined by complete resolution
of periapical bony radiolucency, vary widely from 31% to 100% (Ng et al 2007) but even
where guideline standard treatment is performed, the mean success rates only reach 75%.
The respective mean success of root canal retreatment reaches a similar 77% (Ng et al
2008). The presence of pre-operative infection strongly influences success rates (Ng et al
2008), whilst treatment procedure factors have lesser impact, indicating considerable room
for improvement. The relative lack of sensitivity of radiographic assessment to detect subtle
signs of inflammation around root apices underestimates the true prevalence of persistent
periapical disease. Technological developments over the last 20 years have considerably
enhanced dentists’ ability to mechanically sculpt the internal tooth form to allow the delivery
of fluid irrigants to wash out residual pulp tissue, planktonic bacteria, biofilm complexes and
to kill remaining bacteria. The treatment-related factors having the most profound influence
on periapical healing were accuracy of extension of instrumentation to the apical canal termini,
the degree of bacterial load reduction (dependent on the chemical nature of the irrigant) and
control over root filling and restoration placement (Ng et al 2008). The outcome studies
provide strong inferential evidence that the main factor hindering success is the inability of
treatment protocols to satisfactorily and predictably reach and control the apical infection.
Given the inability of metal instruments to directly plane the walls of the complex internal
surface geometry of teeth, the key issue is the ability of antibacterial fluids to reach this space
and surfaces to effect bacterial biofilm removal.
The science to understand the process of irrigant delivery already exists, but is not fully
exploited by clinical dental practice. The aim of this inter-disciplinary review is to integrate the
science that underpins the physical behaviour of fluid within root canals with current clinical
expertise. The relevant literature, in the dental and fluid dynamics disciplines is reviewed
to derive an informed synthesis of the knowledge and its remaining deficiencies. A further
aim is to allow fluid engineering expertise to inform and re-define key clinical parameters, to
facilitate appropriate further development of the treatment procedure; as such the structure of
the remaining review may seem at variance with traditional clinical dogma and is structured
as follows.
Topical Review
(1)
(2)
(3)
(4)
(5)
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The structure of teeth and the clinical context.
The role and efficacy of irrigation during various stages of root canal preparation.
Understanding clinical approaches to irrigation.
Application of physical principles to clinical procedures.
Concluding statements.
2. Structure of teeth and the clinical context
2.1. Structure of teeth and the root canal system
Teeth are formed by foci of specialized embryonic tissue, a part of which becomes encapsulated
by the formed tooth and is then called the dental pulp. Apart from the formative function of the
specialized cells (odontoblasts), the dental pulp is a highly vascular connective tissue that serves
a protective function helping to defend the tooth from bacterial, chemical, and mechanical
assault. The outer odontoblastic layer secretes the mineralized hard tissue, dentine, which
forms the bulk of the tooth. The odontoblasts interface with its secreted shell by virtue of
their extension of a cellular process into its respective secreted tubule. Dentine is therefore
a specialized connective tissue that contains thousands of tubules radiating outwards from
the dental pulp to the enamel in the crown and the cementum in the root, each with its own
odontoblastic process. The dentinal tubules make up 20–30% of the volume of dentine. The
diameter of the tubules is about 3 μm near the pulp and less than 1 μm peripherally (Gulabivala
2004) (figure 1(a)).
The average root canal system volumes (table 1), corono-apical lengths (table 2) and
apical diameters (table 3) indicate their minute proportions without conveying the enormous
complexity and irregularity of the internal form, which can be seen in figure 2 for a typical
tooth: the root canals are curved, pointed, non-circular, have irregularly varying cross-sections
(figure 3) and side channels and chambers (figure 4). The characteristics of root canals are
variable and are dictated by racial origin, tooth type, disease history and age (Gulabivala et al
2001, 2002, Ng et al 2001, Alavi et al 2002).
2.2. Nature of the disease
Normal wear and tear of teeth in function, disease processes (tooth decay and tooth surface
loss) and dentists’ attempts to repair the resulting defects lead to progressive inflammation of
the dental pulp, sometimes leading to its death, leaving a zone inside the teeth that is incapable
of defending against the entry of bacteria from the mouth. Such infection may lead to abscess
Table 1. Mean volumes (mm3) of dental pulp cavities (data extracted from Fanibunda (1986)).
Tooth type
Maxillary
Mandibular
Central incisor
Lateral incisor
Canine
First premolar
Second premolar
First molar
Second molar
Third molar
12.4 (±3.3)
11.4 (±4.6)
14.7 (±4.8)
18.2 (±5.1)
16.5 (±4.2)
68.2 (±21.4)
44.3 (±29.7)
22.6 (±3.3)
6.1 (±2.5)
7.1 (±2.1)
14.2 (±5.4)
14.9 (±5.7)
14.9 (±6.3)
52.5 (±8.5)
32.9 (±8.4)
31.1 (±11.2)
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Topical Review
(a)
(b,ii)
(b,i)
Figure 1. (a) Ground section of the crown of a tooth showing its dentinal tubular structure.
(b) SEM images of the internal surface of a root canal. In (b(i)) the root canal surface is instrumented
and covered by a smear layer, while in (b(ii)) the smear layer has been dissolved. The inner (pulpal)
diameter of the tubules is about 3 μm.
Table 2. Average corono-apical lengths of teeth (mm) (data extracted from Walker (2004)).
Tooth
Maxillary
Mandibular
Central incisor
Lateral incisor
Canine
First premolar
Second premolar
First molar
Second molar
22.5
22.0
26.5
20.6
21.5
20.8
20.0
20.7
21.1
25.6
21.6
22.3
21.0
19.8
and cyst formation in the periradicular tissues (ligament and bone surrounding the root) at
the junction where the intra-radicular space communicates with the peri-radicular tissues,
principally but not exclusively at the root tip. The disease may cause pain and gum and/or
facial swelling. Pulpal (inside tooth) and periapical (outside tooth) diseases are therefore
Topical Review
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(a)
(b)
Crown
Root - coronal
Root - middle
Root - apical
Figure 2. (a) Photograph of an extracted tooth. The relatively simple canal anatomy in this case
is shown in (b), after the tooth has been rendered transparent by a process called ‘clearing’.
Table 3. Diameters (μm) of apical foramina by tooth and root type (data extracted from Morfis
et al (1994)).
Tooth/root type
Diameter of apical foramen (μm)
Maxillary incisors
Mandibular incisors
Maxillary premolars
Mandibular premolars
Maxillary molars
Palatal root
Mesial root
Distal root
Mandibular molars
Mesial root
Distal root
289.4 ± 120.9
262.5 ± 190.2
210.0 ± 170.9
368.3 ± 183.9
298.0 ± 62.1
235.1 ± 101.0
232.2 ± 66.09
257.5 ± 343.3
392.0 ± 77.5
inflammatory responses to bacterial infection inside teeth. In biological terms, the transition
from pulpitis to apical periodontitis represents a continuum of inflammation spread from
inside the tooth to the surrounding ligament and bone. The transition from one to the other is
therefore marked by the loss of alveolar bone around the canal exits, radiographically evident
as periapical radiolucency. The infection is polymicrobial in nature, with a variable diversity
(Bergenholtz 1974, Wittgow and Sabiston 1975, Sundqvist 1976) and complex, adherent,
community structure known as a biofilm. The biofilm is a bacterial layer on the internal
dentine wall of variable thickness and composition and is abutted by dying or necrotic pulp
tissue or migrating host defence cells, polymorphs and macrophages (Nair 1987, Richardson
et al 2009). The hydrated biofilm is composed of bacterial cells, variable amounts of a
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(a)
(b)
Figure 3. (a) Root canal instrument negotiating the length of a gently curved root canal. The
cross-sections of un-instrumented root canals are shown in (b). The cross-section of the root is
generally non-circular.
polysaccharide extracellular matrix, metabolites and other minor excreted products. The
bacterial biofilm may also penetrate into dentinal tubules by a variable distance but not all
tubules are so invaded (Love and Jenkinson 2002).
2.3. Role of intervention
The clinical aims of root canal treatment have been described by the European Society of
Endodontology (2006). Root canal treatment is the dental procedure used to either (1) prevent
apical periodontitis by early treatment of diseased or infected soft tissue contained in the
hollow space inside teeth or (2) help resolve apical periodontitis, when the dental pulp is
necrotic and the hollow space infected. Root canal ‘retreatment’ may be performed to treat
persistent ‘post-treatment’ apical periodontitis due to treatment failure (European Society of
Endodontology 2006). The complicating feature of the treatment is that the biofilm-coated
internal surface has an enormously complex and irregular spatial configuration.
Root canal treatment requires access to this complex internal surface and space and it
begins with entry to the internal aspect of the tooth by drilling through the top of the crown
(figures 2 and 3). The narrow extensions of the space into the roots, called canals, are
accessed by specialized metal instruments (files, reamers (figure 5(a))). These are used to
enlarge and sculpt the canal space into a tapering form, sufficiently to allow disinfecting
agents (fluids) to reach and have contact with its entire surface. The irrigants and irrigation
have multiple roles, some of which dominate at different stages of the treatment procedure. In
order to effectively reach these surfaces, the fluid has to adequately flush out the canal space
of residual pulp tissue and dentine and bacterial debris, which may even have been compacted
Topical Review
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(a)
(b)
Figure 4. (a) Photograph of a cleared tooth showing complex canal anatomy filled with Indian ink.
(b) Schematic of the tooth in (a). The central tapering channels represent the instrumented canals
within the original root canal anatomy and through which irrigant penetrates to the peripheries of
the root canal system chamber (indicated by arrows).
and displaced into the outer and distal lying spaces of the root canal system. One of the
roles of the irrigant at this stage is to help dissolve the residual organic tissue and bacterial
biofilm. As the instruments do not plane the entire irregular root canal system surface, 30–50%
remaining un-instrumented, one of the roles of the irrigant is to help remove the residual biofilm
from the un-instrumented surface and another is to remove the smear layer formed over the
instrumented surface (a property of the hydroxyapatite which forms the mineral component)
(figure 1(b)). The antibacterial fluid delivered into the canals must retain its potency as it
reaches the target to facilitate the removal and killing of the bacteria. Once the canal system is
considered to be sufficiently decontaminated (judged by bacterial culture tests and/or absence
of signs and symptoms of disease), it is filled with an inert material such as gutta-percha
together with a paste-like sealer to prevent bacterial recontamination. This process of finding,
placing instruments into, enlarging, irrigating and filling the root canal space consists of a
series of highly skill-dependent and inter-dependent procedures, taking time and patience.
The outcome of the treatment is monitored clinically and with radiographs to ensure that the
periapical tissues heal. The healing involves bone repair that can take anything from 6 months
to 4 years and sometimes longer, probably because of the residual apical infection (Wada
et al 1998, Nair et al 2005, Ng et al 2008).
3. The role and efficacy of irrigation during various stages of root canal preparation
The irrigant and irrigation fulfil multiple roles during root canal preparation, and although many
of these processes occur simultaneously, conceptually one of these functions of irrigation takes
on a dominant role in each of the different phases of root canal preparation.
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Topical Review
(a,i)
(a,ii)
(a,iii)
(b)
Figure 5. Summary of the instruments used in root canal treatment. In (a), SEM images show
the tips of three endodontic files (a(i) file; a(ii) flexofile; a(iii) reamer). In (b) a variety of needle
types are shown. From left-to-right, these are flat open-ended (FOE), bevelled open-ended (BOE),
side-cut open-ended (SCOE) (Monojet, Sherwood Medical, St Louis, USA), closed-end single
side-opening CE-SSO (Endovage Dental from p 9), respectively. The recommended needle types
are the last two designs.
3.1. Irrigant role and delivery and their relationship with the mechanics of canal preparation
Root canal treatment may be divided into three distinct phases by the dominant role of
irrigation.
3.1.1. Canal negotiation phase. After initial entry into the tooth via the access cavity, the
clinical goal is to locate and negotiate the identifiable canals using instruments small enough
to achieve penetration to their full length. At this stage, the irrigant may only be delivered
predictably as far as the pulp chamber or coronal third of canal depending on the extent of
penetration of the irrigating needle (figure 5(b)). A clinical tactic to enable further direct
apical penetration of the irrigant at an earlier stage in canal preparation involves pre-flaring
of the upper part of the canal (coronal preflaring or crowndown approach). The irrigant may
facilitate such canal negotiation and equally the act of file negotiation may help to drag the
fluid interface further apically through surface tension effects. An initial problem to overcome
may be the presence of gas bubbles ahead of the advancing front of the irrigant; this has been
described as the ‘vapour lock effect’ by Gu et al (2009). Agitation with a file may help break up
the gas bubbles, as may the use of liquids with lower surface tension. Some clinicians believe
that lubricating gels may help facilitate penetration of the instruments (Buchanan 1991).
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3.1.2. Canal enlargement phase. Once the main canals have been negotiated to their full
verified lengths, the mechanical enlargement or shaping of the canals can be undertaken.
Mechanical shaping or enlargement of the canal at the expense of the dentine may help
free up tags of pulp tissue and will also generate dentine debris, which is released into the
fluid reservoir within the canal cavity. As the metal instruments negotiate and enlarge the
canal space, it progressively becomes easier to inject the fluids further apically into the canal
system. The primary role of the irrigant at this stage is to flush out this debris with the frequent
injection of irrigant in between filing and reaming. If the files are inserted to the full length of
the canal from the outset, dentine debris is generated throughout the length of the canal but the
replacement of the spent, debris-laden irrigant is only effective in the coronal part of the canal
where the irrigant needle is able to reach (the difficulty of accessing the apical region of the root
canal is clear in figure 3(a)). The dissolving organic pulp contents mix with the dentine debris
and may lead to the progressive increase in the viscosity of the irrigant fluid, to the point where
it may begin to behave as a paste, clinically known as dentine ‘mud’ and be pushed by the
piston action of the increasingly larger files into the as yet un-debrided and still contaminated
apical anatomy. This process leads to apical blockage of the canal and eventually, forcing of
metal instruments by the clinician in the hope of regaining length; this forcing of instruments
may in turn cause ‘transportation’ (deviation of the instrumented channel from the original
canal) and uncontrolled shaping of the canal walls. To prevent this problem, the suspended
debris is flushed out by delivering the irrigant as far apically into the canal as possible with a
needle but during the initial phases of canal preparation, deposition of the irrigant sufficiently
apically is limited by the lack of access of the irrigating needle to near the terminus of the
canal. To help flush out debris from the apical part, the freshly replaced coronal irrigant is
mixed with the still debris-laden fluid in the apical part by apical transference and mixing
using a small file, a process called ‘recapitulation’ (figure 6). Passing the file beyond the canal
terminus to prevent its blockage is called ‘patency filing’. The frequency of replenishment of
the coronal irrigant, which with progressive canal preparation allows irrigant delivery further
and further apically, as well as the frequency of recapitulation, is crucial in the ability to
maintain the apical canal anatomy patent (Buchanan 1991).
3.1.3. Active canal irrigation phase. Once the canal shaping is complete as judged by
the achievement of the pre-defined continuous tapering form, the entire root canal system
(including the instrumented and un-instrumented surfaces) should be accessible for final
washing by the irrigant. The irrigant is again delivered by injection through a needle into
the prepared part of the canal and the process completed by means of agitation, pumping and
mixing to drive the irrigant into the ‘unshaped’ part of the canal system (figure 4). This final
phase of irrigation is relatively poorly adopted in general dental practice because of the lack
of awareness of its importance. The actual practice may vary in terms of volume of irrigant
used, the nature of agitation accompanying it and the duration of soak-time employed. Some
practitioners will also alternate between a number of chemically distinct solutions in the belief
that the practice synergizes the chemical effects.
The dominant roles of the irrigant and irrigation in the three phases may therefore be
summarized as follows:
(1) to lubricate the insertion and negotiation of root canal instruments to the root canal
termini;
(2) to flush out the root canal system of loosened debris generated during root canal filing or
reaming and
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Topical Review
(a)
(b)
Figure 6. Simulated prepared root canal in an EndoVu block, which has the same physical
dimensions as a tooth. Prepared root canal diameter varies from 0.9–2.0 mm coronally to 0.3 mm
at the root apex. In (a), water is injected through a 27-gauge needle into a suspension containing
debris generated by filing the canal. In (b) the fluid column is mixed by the vertical agitation of a
file (‘recapitulation’) which extends the whole length of the canal.
(3) to dissolve and disrupt bacterial biofilm and pulpal tissue, kill bacteria and denature toxins
to the outer-most reaches of the root canal system.
3.2. Efficacy of root canal treatment procedures
A variety of outcome measures in both laboratory and clinical studies have been used to
test the efficacy of mechanical instrumentation in conjunction with organic tissue-dissolving,
antibacterial and neutral irrigating agents.
3.2.1. Neutral irrigation solutions (saline, water). Neutral solutions in conjunction with
mechanical preparation are insufficient to debride canals free of pulp tissue, debris, smear
layer or bacterial content. The assessment is often divided into coronal, middle and apical
portions of the root. As a general observation, given the tapering shape of the canals, and the
need to remove more dentine coronally than apically, the greatest cleaning effect is evident
coronally and the minimal effect is evident apically (Walton 1976, Biffi and Rodrigues 1989,
Baumgartner and Cuenin 1992, Evans et al 2001).
Monitoring changes in root canal bacterial load and relative proportion of bacterial species
following each stage of root canal treatment has been a valuable tool for testing the efficacy
of such procedures even though the available methods require an additional laboratory step.
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Both reduction in bacterial cell numbers and the proportion of teeth with negative cultures
have been used as surrogate outcome measures.
The most influential microbiological evidence for contemporary root canal treatment
was provided by a series of studies from Sundqvist and colleagues (Byström and Sundqvist
1981, 1983, 1985, Byström et al 1985, Sjögren and Sundqvist 1987, Sjögren et al 1991).
They evaluated the effect of various root canal treatment procedures on the microbiota, both
qualitatively and quantitatively, using standardized methodology. Mechanical preparation
with water or saline irrigation proved to be the least effective way of reducing the bacterial
load and achieving negative cultures.
Considering a range of studies collectively, negative cultures were achieved after using
water or saline irrigation in 4.6%–75% (mean of 25%) of cases (Ingle and Zeldow 1958,
Nicholls 1962, Grahnén and Krasse 1963, Zeldow and Ingle 1963, Byström and Sundqvist
1981, Ørstavik et al 1991, Dalton et al 1998). Taking account of these data together with
the fact that mechanical instruments only plane up to 61% of the canal surface (Mannan et al
2001, Peters et al 2001), the considered role of canal preparation has undergone a shift from
one of fulfilling a prime debriding function to one regarded more as a radicular access for the
irrigant and root-filling materials to the complex root canal systems (Gulabivala et al 2005).
An important role of root canal irrigation is to help remove the residual bacterial biofilm from
the inaccessible, un-instrumented surfaces.
3.2.2. Antibacterial irrigation solutions (sodium hypochlorite, chlorhexidine gluconate,
Biosept). The role of antibacterial irrigants is to help reduce the bacterial load in the root canal
system. Bacterial load reduction is improved by the addition of sodium hypochlorite (0.5%)
irrigation and further by alternate irrigation with ethylenediaminetetraacetic acid (EDTA) and
5.0% sodium hypochlorite solution (Byström and Sundqvist 1983). The irrigation regimes
reduce the residual bacterial load from an initial range of 102–108 cells per unit sample to
102–103 cells after initial debridement. Based collectively on a range of studies, the frequency
of negative culture (25%–98%) (mean of 75%) increases substantially when sodium hypochlorite (concentration of 0.5%–5.0%) is used as the irrigant during mechanical preparation (Cvek
et al 1976, Byström and Sundqvist 1981, 1983, 1985, Sjögren and Sundqvist 1987, Yared and
Bou Dagher 1994, Gomes et al 1996, Shuping et al 2000, Peters et al 2002, Kvist et al 2004,
Vianna et al 2006, Siqueira et al 2007a, 2007b, 2007c). The wide range of outcomes may be
attributable to variation in the pre-operative condition of the teeth, the concentration, volume
and method of delivery of the irrigant solution, the size and shape of the root canal system, as
well as the microbiological methods used for detection.
The findings on the influence of size of apical preparation on bacterial load reduction have
been inconsistent. Some groups have shown more effective bacterial debridement with larger
compared to smaller apical preparation sizes (Parris et al 1994, Rollison et al 2002, Card et al
2002), whereas others have failed to show a difference (Yared and Bou Dagher 1994, Coldero
et al 2002, Wang et al 2007). The discrepancies may again be attributed to the factors listed
above, but there is a lack of clarity in the literature about the precise reasons.
The additional use of ultrasonic agitation of the sodium hypochlorite irrigant (Sjögren
and Sundqvist 1987, Carver et al 2007) and alternate irrigation between EDTA and sodium
hypochlorite (Byström and Sundqvist 1985) during mechanical preparation achieves an even
higher frequency of negative bacterial cultures. The adoption of the chlorhexidine gluconate
(concentration of 0.12%–2.5%) solution as an antibacterial agent for irrigation was suggested
10 years previously (Kuruvilla and Kamath 1998); it had gradually begun to gain acceptance.
However, the number of negative cultures achieved using chlorhexidine irrigation was similar
(Waltimo et al 2005, Siqueira et al 2007c) to or lower (Vianna et al 2006) than that
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achieved with sodium hypochlorite irrigation; a finding also corroborated in retreatment cases
R
, a quaternary ammonium compound, used for irrigation
(Schirrmeister et al 2007). Biosept
in the 1960s gave 32% (Grahnén and Krasse 1963) and 40% (Engström 1964) negative cultures,
respectively.
The adoption of multiple chemical agents to irrigate the root canal system has resulted in
the possibility of synergism between them to enhance bacteria killing in the case of NaOCl and
EDTA (Byström and Sundqvist 1985), with the potential of increased clinical success rates
(Ng 2008). It has, however, also raised the spectre of neutralizing chemical reactions between
chlorhexidine and NaOCl (Basrani et al 2007, Bui et al 2008) which reduce their combined
effect and the potential for toxic precipitation and associated reduced clinical success rates
(Ng 2008). Finally, it is worth remembering that the physical mixing of different chemicals as
well as chemical interactions may alter the rheology of the irrigant, thereby altering the fluid
dynamics.
Triangulation of these diverse sources of clinically oriented data indicate that whilst
biofilm disruption and bacterial killing may be relatively easily achieved in laboratory tests
(Spratt et al 2001, Bryce et al 2009), albeit with their own set of variations, it is relatively
more difficult to achieve the same within tooth infection models (Gulabivala et al 2004) and
in teeth, clinically. It is intuitively evident that a principal reason for this is the problem of
delivery of irrigant agents to the complex apical anatomy, where reside the last vestiges of the
root canal biofilm (Nair et al 2005, Ng et al 2007, 2008).
3.3. Irrigant extrusion into periapical tissues
Ironically, although the chief problem of irrigation is irrigant delivery to the complex apical
anatomy of the root canal system, it is likely that in most cases there may be some seepage
or diffusion of the spent irrigant into the periapical tissues (Martin and Cunningham 1982),
which probably causes little or no harm. Very occasionally extrusion of a sufficient volume of
irrigant into the periapical tissues may cause adverse reactions, particularly when using NaOCl
(Hülsmann and Hahn 2000). Studying the causes of extrusion is very difficult in the clinical
situation with reliance having to be placed on surrogate outcome measures or alternatively on
the use of in vitro models.
Extrusion of NaOCl irrigant during routine canal instrumentation using dynamic syringe
irrigation (achieved with a 23-gauge needle extending as deep as possible into the canal without
binding) has been found to be significantly worse than when using static irrigation (achieved
by flooding the access cavity with irrigant) (Brown et al 1995). Using a different outcome
measure, an overall reduction in post-operative discomfort was found when using needles with
closed-end, multiple side-openings for irrigation (Goldman et al 1979). Positioning of the
needle appears to have an important effect on extrusion (Williams et al 1995).
The potential for extrusion when using negative pressure, sonic or ultrasonic systems
has been investigated using extracted tooth models with pre-prepared canals. Irrigant was
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system, but a minimal amount
found not to be extruded using the negative pressure EndoVac
TM
was extruded using the sonic EndoActivator . Both these techniques had significantly
R
less extrusion than manual dynamic irrigation, automated dynamic irrigation (RinsEndo
handpiece) or ultrasonic active irrigation (Desai & van Himel 2009). Williams et al (1995)
and Hauser et al (2007) found that although irrigant delivered through the RinsEndo handpiece
penetrated deeper into the dentinal tubules, there was also a higher risk of apical extrusion
than irrigant delivered manually through syringe irrigation. Ultrasonic irrigation during canal
preparation was found to result in significantly more extrusion than dynamic syringe irrigation,
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although extrusion could be reduced by limiting the extension of the ultrasonic file to 1 mm
short of the canal terminus (Martin and Cunningham 1982).
Interestingly, and counter-intuitively, enlarging the apical terminus of the canal reduced
extruded material, but it is highly likely that this was due to the use of instruments short of the
enlarged canal termini (Lambrianidis et al 2001). The finding was corroborated in a separate
study where size of apical foramen was not found to have a significant association with the
amount of extruded irrigant (Williams et al 1995).
Clinically, therefore there is a balance to be struck between effectively delivering the
irrigant to the complex apical anatomy, whilst preventing its extrusion periapically.
4. Understanding clinical approaches to irrigation
4.1. Irrigant actions in clinical operations
Irrigant is introduced into a root canal system during and after canal preparation in order for it
to be dispersed throughout and to help disinfect its surfaces and space, to exert shear stress on
the walls to remove the adherent biofilm and smear layer, and to remove from the canal cavity,
debris generated by instrumentation. These objectives may be satisfied through developing
equipment and techniques for the introduction of irrigant and for its agitation. There are two
separate processes by which these objectives are attained: first the dentist delivers the fluid
and then as a separate step agitates it in situ using either the needle or another instrument
(including sonic and ultrasonic devices).
4.1.1. Methods for the introduction of irrigant. Irrigant is commonly delivered continually
from a syringe through a needle. The key variables are the rate of irrigant delivery (Q), which
varies tremendously amongst operators (Q = 0.01–1.25 mL s−1) (Boutsioukis et al 2007a)
and the volume of irrigant (V) used for each treatment (V = 6–42 mL) (Falk and Sedgley
2005, Nguy and Sedgley 2006, Nielsen and Baumgartner 2007). The method of delivery may
be manual (for instance, using a syringe) or automated (using one of several commercially
available devices). For manual injection, the volume of the syringe used may vary between
3 and 5 mL. There is a tendency for Q to be higher for male than female operators with a
negative correlation between clinical experience and volume of irrigant delivered, regardless
of needle used (Boutsioukis et al 2007a). Automated delivery systems can be categorized into
R
), continuous
several types: continuous delivery of irrigant through one needle (Quantec-E
R
delivery and evacuation achieved through two needles (EndoVac ), pulsating injection from
R
), and alternating delivery and evacuation through outer and inner
one needle (RinsEndo
tubings (non-instrumentation technique (NIT)), respectively.
Of central importance to the introduction of irrigant is the size and design of the needle
used. The internal dI and external dE diameters of needles are usually expressed in terms of
standard gauge sizes and are presented in table 4, along with their range. The internal and
external needle diameters decrease as the gauge number increases. A survey of endodontic
irrigation needles revealed that few needles complied with the ISO nominal size but all were
within tolerance limits (Boutsioukis et al 2007b). Gauge 23 or 25 needles were widely used
before the 20th century, whereas the current emphasis is to use gauge 27 or 30. Root canals
are typically 0.5–2 mm in diameter at the crown and taper down to less than 0.3 mm in mature
roots. A 27-gauge needle will therefore penetrate to the apical third at the end of the canal
preparation. Increasing the needle gauge has two main effects on clinical treatment. First,
smaller needles are able to penetrate deeper into the root canal and secondly, for equivalent
delivery rates of irrigant, the viscous effects (elaborated further in section 5.2) are weaker for
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Table 4. Medical stainless steel needle specifications according to ISO 9626:1991/Amd.1:2001
(ISO 9626 2001) (adopted from Boutsioukis et al (2007b)).
Designated metric
Range of external
diameters (mm)
Internal
diameter (mm)
Gauge size
size (mm)
Min
Max
Min
21
23
25
27
30
0.8
0.6
0.5
0.4
0.3
0.800
0.600
0.500
0.400
0.298
0.830
0.673
0.530
0.420
0.320
0.490
0.317
0.232
0.184
0.133
narrower needles because the flow is faster. Both contribute to more effective removal of debris
and bacteria (Baker et al 1975, Abou-Rass and Piccinino 1982, Sedgley et al 2005). However,
finer needles require increased effort to inject the irrigant and result in higher pressure within
the barrel, up to 400–550 kPa (Boutsioukis et al 2007b).
Commercially available irrigation needle tips are modified so that the exit does not face
the canal terminus and remains blunt in order to reduce the risk of canal binding and irrigant
displacement into the periradicular tissues. The range of different needle designs available
is shown in figure 5(b); they are categorized as flat open-ended (FOE), bevelled open-ended
(BOE), side-cut open-ended (SCOE) and those with closed-end and single side-openings
(CE-SSO). The recommended types are the SCOE and CE-SSO (the last two designs in
figure 5(b)) (Moser and Heuer 1982). Other types are available such as a needle with a closedend and multiple side-openings (National Patent Development, NY, USA, Goldman et al
1976).
In the two-needle irrigation systems, the finer evacuation needle penetrates further than
the delivery needle; the needle gauges are 30 and 27, respectively. The ends of delivery and
evacuation needles are placed 3 and 12 mm from the apical end of the root, respectively.
Irrigant is delivered at a typical rate of 0.05 mL s−1 (Fukumoto et al 2006). In pulsating
injection systems, such as the RinsEndo, the flow rate is 0.1 mL s−1 and is pulsed with a
frequency of 1.6 Hz. The main difference between the commercial systems is the form and
size of the needle type for evacuation of irrigant at the apical portion of the canal, as well as
the flow rates.
4.1.2. Agitation of irrigant. Agitation takes place by ‘activating’ irrigant through the
introduction of an instrument into the canal and moving it within the canal with a reciprocating,
oscillating or rotating action so that the irrigant is dispersed and debris is removed from the
extremities of the canal. There are several methods by which this can be achieved (figure 7):
(1) manual reciprocation of the instrument within a canal by the dentist; (2) sonic or ultrasonic
activation to achieve oscillation of the instrument and (3) introduction into the canal of a
mechanically driven rotary instrument.
4.2. Measuring the effectiveness of irrigation regimens
Many methods have been used to measure the effectiveness of various root canal irrigation and
therefore treatment procedures. Clinical trials and ex vivo models are attractive and relevant
as they deal with the actual organ or problem in question (the material and shape of the canal
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(b,i)
(a)
(b,ii)
dI
Average
velocity in
needle, U N
dE
Speed of
cone into the
canal, U A
Canal
diameter, D
streamlines
Canal
diameter at
needle end,
D
Diameter of
agitator, d A
Enhanced
diffusion
caused by
shear
dS
Stagnation
plane
Distance
from end of
root x
Root to end
of agitator
( lN )
Figure 7. Schematic diagram of geometry and notation based on experimental results for:
(a) injection of irrigant into the root canal by a single needle; and (b) agitation by (i) the vertical
motion of a file or gutta-percha cone or (ii) in plane displacement of instrument by ultrasonic
vibration. In b(ii) ↔ = direction of oscillation
are particularly important). The latter models also allow measurement of the state of the
canal before and after treatment. The clinically relevant outcome measures used to evaluate
the effectiveness of treatment include measurements of: periapical healing (Ng et al 2008),
tooth survival (Ng et al 2010), quantitative and qualitative changes in the cultivable microbiota
(Byström and Sundqvist 1981) and residual bacterial biofilm (Nair et al 2005).
Ex vivo models have been used to gain additional mechanistic insight through assessment
of debris, smear layer or bacterial reduction (Gulabivala et al 2005). Such measurements,
whilst clinically important and relevant, do not shed light on the transient physical or chemical
phenomena leading to the final outcome. Furthermore, data from clinical or ex vivo studies
may be difficult to interpret and compare. The uncontrolled variables inherent in such systems
also complicate evaluation of the effect of canal systems, teeth and patients on outcome. As an
example, the volume flux of the irrigant delivered varies by more than a factor of 50 between
different studies. These difficulties can be alleviated by using simplified in vitro models
(where the canals are simulated by holes in an artificial medium such as Perspex), which
allow the canal geometry to be specified and controlled. Furthermore, such models also allow
physical or chemical outcome measures to be tracked with a suitable frequency, or indeed
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continuously to allow depiction of the physical processes. Unfortunately, discrepancies in the
results between in vitro or ex vivo models and clinical trials may lead clinicians to discount
the former as being irrelevant to the complexity of natural systems. The interpretation
of data from simple systems and their application to progressively more complex systems
require a fundamental scientific insight, in order to achieve a synthesis of outcomes that
could model and help explain the physical behaviour in real systems. Nowhere are these
problems more evident, and the principles more pertinent, than in the literature on root canal
irrigation.
A wide variety of methods using a combination of real, simulated or theoretical canal
models have been developed to assess the effectiveness of needle designs and agitation
methods. The effectiveness of needle design and placement has been assessed using the
fraction of irrigant in the root canal being replaced (Ram 1977), degree of penetration of the
irrigant (Goldman et al 1976, Khademi et al 2006) or removal of canal contents or wall surface
coatings as outcome measures. The latter have included flushing out of artificial particles
(Chow 1983), debris (Goldman et al 1979, Abou-Rass and Piccinino 1982) or bacterial cells
(Falk and Sedgley 2005) from the canal space and removal of simulated ‘biofilm’ (Huang
et al 2008) and smear layer (Goldman et al 1979) from the root canal surfaces. These methods
have also been used to measure the effect of different root canal irrigation variables on selected
outcomes, including the effect of agitation.
In order to tackle the problem systematically, it helps to visualize the full extent of the
root canal system problem in its three-dimensional complexity and scalar dimensions and to
resolve it into its simpler component parts. The accuracy of the complex depiction at the one
extreme and the resolved simplicity at the other end should improve the probability of revealing
innate mechanisms underpinning any interaction between the simpler component parts. As
an example, it is thought that agitation encourages the mixing of root canal fluids, allowing
freshly injected irrigant to replace the resident fluid already occupying the canal (figures 6
and 7); it may also facilitate penetration of the irrigant into lateral canals and chambers beyond
the prepared canal (figure 4). The intended outcome of the agitation should be to disperse
newly introduced irrigant throughout the root canal system and also to dilute the resident
debris by flushing it out through its displacing motion. The effectiveness of this process will
be dependent upon the extent of the fluid volume agitated within the cavity, as well as the
transport processes used to place or remove the bolus of fluid. In addition, it is also hoped
that the stresses generated by the agitation would encourage the removal of material bound or
adherent to the canal walls. Measurement of the ultimate clinical outcomes is insufficient to
reveal the processes that lead to the clinical outcomes. It is therefore necessary to specifically
identify and demarcate the nature of subordinate physical processes likely to influence and
improve the clinical outcomes. In order to achieve this aim, expertise and insight from the
physical and engineering sciences can be harnessed to elucidate the nature of fundamental
fluid-flow processes inherent in such systems.
4.3. Reported effectiveness of irrigation
There have been many ex vivo studies of the effectiveness of different irrigation regimens
utilizing various injection needle designs and techniques.
4.3.1. Irrigant introduction. The difficulty in reconciling the outcomes from multiple ex vivo
investigations is illustrated by the results of four separate studies evaluating the effect of various
needle geometries on irrigation effectiveness but using different outcome measures: irrigant
penetration (Goldman et al 1976); efficacy of bacterial removal (Vinothkumar et al 2007);
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R65
efficacy of in situ smear layer and debris removal (Goldman et al 1979) and efficacy of artificial
debris removal (Moser and Heuer 1982). Goldman et al (1976) found that using a closed-end
needle with multiple side-openings (CE-MSO) placed within 2 mm of the apex produced a
more widespread and uniform distribution of the irrigant in the canal than using a closed-end
needle with a single-side opening (CE-SSO). Consistent with this observation, Boutsioukis
et al (2009) studied the irrigant flow pattern within a prepared root canal computationally and
reported that the irrigant flow pattern for the CE-SSO needle was influenced by the flow rate
but the irrigant replacement was limited to 1–1.5 mm apical to the needle tip, regardless of
the flow rate. Goldman et al (1979) used scanning electron microscopy to examine the extent
of residual in situ smear layer and debris during serial instrumentation of canals in freshly
extracted teeth; they concluded that irrigation with the sodium hypochlorite solution using
the CE-MSO needle was more effective than using the conventional 23-gauge needle with a
BOE. In contrast, using simulated canals, Moser and Heuer (1982) showed that a CE-MSO
needle was less effective in removing artificial debris than a SCOE needle, where in both cases
the needle was 23 gauge. More recently, Vinothkumar et al (2007) apparently contradicted
Moser and Heuer (1982) by showing that CE-SSO 25-gauge needles were more effective
than CE-MSO or BOE needles in removing bacteria from root canals with a specified size
(apical size 0.60 mm; 0.04 taper). The contradictions could be attributed to differences in
a number of factors, including test model design, size and taper of the canal, size of needle
and the rate of irrigant delivery. Irrigant delivery was standardized at a rate of 0.2 mL s−1 by
Vinothkumar et al (2007) but was unspecified by Moser and Heuer (1982), however, they did
report that the average time necessary to empty a 2 to 3 mL syringe with the approximation
of the forces used clinically was 60 s, a much slower rate than that used by Vinothkumar
et al (2007). These examples illustrate the benefit of comprehensive data recording of inherent
variables to account for differences, always assuming that the likely influencing factors had
been anticipated and incorporated into the study design.
An example of an ex vivo study attempting to record data comprehensively is that of
Huang et al (2008), who undertook a systematic evaluation of the influence of canal size
and geometry (apical size 0.20 or 0.40 mm, taper 0.04 or 0.08) and irrigant volume on the
fraction of simulated biofilm removed. A CE-SSO needle was used with the direction of
the single-side opening location fixed in all the tests on single-rooted extracted teeth with
single canals. The bacterial biofilm was simulated using dyed rat-tail collagen (First Link Ltd,
Birmingham, UK), which was applied in multiple layers to the root canal walls. The 30-gauge
needle was inserted to 4.5 mm short of the root apex and delivered the sodium hypochlorite
solution at a rate of 0.1 mL s−1. After every 9 mL of irrigant had been delivered (up to a
total of 36 mL), the percentage of the canal surface covered with residual stained collagen
was measured from the images. It was found that the efficacy of biofilm simulant removal
was improved by increasing the apical size and taper of the canal, the volume of irrigant used
and the orientation of the side opening of the needle (Huang et al 2008). The percentage of
canal surface coverage with residual collagen increased from the apex coronally. Complete
removal was not achieved in any of the samples. The finding on the effect of canal preparation
size was consistent with the report by Falk and Sedgley (2005) who used the alternative
methodology of measuring removal of artificially introduced bioluminescent bacteria from
extracted teeth. Using the same test model, Nguy and Sedgley (2006) showed that canal
curvature and size also greatly affected irrigation. For canals with a curvature of less than
20◦ , the canal size had no effect on irrigation efficacy, but this increased significantly with
canal size where the canal curvature was between 24◦ and 28◦ . In contrast, the efficacy of
removal of artificially placed debris in the canal space was not affected by the apical size
(0.25 versus 0.40 mm in diameter) of flared canal preparations (Abou-Rass and Piccinino
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1982). The features of these studies that allow appropriate interpretation and intuitive synthesis
include control of canal topology, the use of a direct physical measurement and the use of
an outcome measure (e.g., erosion of the wall-adherent film) that could be tracked over
time.
Similar ex vivo extracted tooth models have been used to study the effect of commercially
available automated systems for introducing irrigants. There is evidence that such automated
systems can be superior to the conventional syringe/needle systems for removal of pulp tissue
(Braun et al 2005), killing of Enterococcus faecalis (Muselmani et al 2005), removal of
dyed collagen film from the canal surface (McGill et al 2008) and removal of a smear layer
(Fukumoto et al 2006). The commercial systems show an improvement in debris removal
within 1 mm from the root apex, but there was no observable improvement 3 mm from the apex,
as compared to syringe injection (Nielsen and Baumgartner 2007). Again, these studies suffer
from the measurement of final effects only and give little direct insight about the processes
that could be manipulated to effect improvements. Measurement of the processes may help to
decipher the features of the automated systems that confer real benefit and help elucidate the
mechanisms involved.
An example of clouded understanding resulting from the lack of such insight into
fundamental operating processes is provided by experiments on the assisted introduction
of irrigant by the ‘non-instrumentation technology’ (NIT) developed by Lussi et al (1993).
This method claimed to deliver the irrigant whilst circumventing the need for mechanical
enlargement of the root canal system. The authors recommended irrigation with the sodium
hypochlorite solution under alternating pressure fields, which they claimed produced hydrodynamic turbulence that enabled perfusion of even the most minute ramifications of the root
canal system. In vitro experiments on NIT canal irrigation with the sodium hypochlorite
solution of freshly extracted teeth with vital pulps apparently resulted in significantly cleaner
apical canals than with dynamic syringe irrigation following conventional canal enlargement
(Lussi et al 1999). The device is putatively still under development (Lussi et al 1995, 1999)
and not available for clinical use. There is an absence of any definitive demonstration of
the claimed physical processes underlying the observed effect in the published literature and
from a physical perspective it would be surprising if the device was indeed able to generate
turbulence throughout such a confined system.
4.3.2. Manual agitation. Gu et al (2009) have recently reviewed the contemporary irrigant
agitation techniques. The simplest method for agitating canal fluid is to introduce an instrument
into the canal and to manually redistribute it along the canal with a reciprocating action.
Appropriately sized instruments or devices are likely to be readily available in practice at
no extra cost. Such agitation of canal irrigant may be achieved using hand files (Cecic et al
1984), irrigation needles (Druttman and Stock 1989), ‘well-fitting’ tapered gutta-percha points
(Huang et al 2008) or brushes (Gu et al 2009).
There is evidence that manual irrigation can be effective. Agitation of the canal
irrigant using hand files or irrigation needles removed significantly more test albumin
(Cecic et al 1984) and allowed better apical irrigant replacement (Druttman and Stock
1989). The notion was reinforced by unpublished data and video footage from simulated
canals in clear plastic blocks by Machtou (2003, personal communication), which graphically
showed that active irrigation (push-pull reciprocal agitation with a well-fitting, tapered
gutta-percha point) can improve the penetration and exchange of irrigant apically as compared
to dynamic syringe irrigation. This was confirmed experimentally by Huang et al (2008)
using a dyed collagen film model where gutta-percha points were typically displaced to
within 1 mm of the terminus of the canal and then retracted at a frequency of 1 Hz. Once
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R67
practiced, this procedure is neither time consuming nor laborious but may be perceived by
busy dentists to be so. It does however have a slightly increased risk of apical extrusion
(Alexander et al 2010).
4.3.3. Sonic and ultrasonic devices. Automated systems specifically designed for agitation
of the irrigant within the root canal system (Gu et al 2009) include sonic (Sabins et al 2003,
Ruddle 2007) and ultrasonic (Cunningham and Martin 1982, Cunningham et al 1982, Sabins
et al 2003) devices. Activation of irrigant with sonic or ultrasonic devices involves the agitation
of either a syringe-delivered bolus of fluid in the canal or by simultaneous delivery (via a custom
reservoir) and agitation. The sonic devices available for irrigant agitation include the Sonic
R
system
Air Endo Handpiece (Micromega 1500, Besancon, France) and the EndoActivator
(Advanced Endodontics, Santa Barbara, CA, USA). The former is driven by air pressure
to produce vibration frequencies (manufacturer data: 1500–3000 Hz) and which also drive
R
stainless steel files to aid simultaneous canal preparation and irrigation. The EndoActivator
is electrically driven and is reputed by the manufacturer to work at the much lower frequencies
of 33, 100 and 167 Hz but a recent study measured the putative frequencies to be 160, 175
and 190 Hz, respectively (Jiang et al 2010). The instrument was designed to use polymer
tips of various sizes (ISO size 15, 25, 35) and tapers (0.02, 0.04) for agitation of the irrigant
(Ruddle 2007) to avoid the potential risks of instrument separation associated with metal files,
ledge formation and canal transportation (Hülsmann and Stryga 1993). Preliminary data on
debris and smear layer removal were promising from Ruddle (2007) but conflicting from UrozTorres et al (2010). The outcome measure of dye penetration into dentinal tubules in the apical
5 mm of the root found ultrasonic activation to be best, manual activation the worst and sonic
activation of intermediate value (Paragliola et al 2010). Further more systematic research is
awaited.
The much higher frequencies of ultrasonic vibration (20–40 kHz) are achieved with either
magnetostrictive or piezoelectric devices. Magnetostrictive transducers produce an elliptical
motion at the working tip, whilst piezoelectric transducers produce longitudinal or transverse
linear motions (Cracknell 1980). Ultrasonic agitation of irrigant allowed better penetration of
irrigant apically than manual syringe irrigation when a viscous irrigant (viscosity approaching
5.8× that of sodium hypochlorite) was used (Teplitsky et al 1987). When the irrigant viscosity
was close to that of sodium hypochlorite, the benefit from ultrasound was only evident in narrow
canals (smaller than ISO size 30), but canal curvature had no influence on irrigant penetration
(Teplitsky et al 1987). The beneficial effect of higher viscosity irrigants during ultrasonic
irrigation was also demonstrated in a preliminary study using the ‘ex vivo dyed collagen film
model’ (Merivale et al 2009) from our group.
A number of approaches have been used to measure the influence of sonic and ultrasonic
vibration on irrigation. The outcome measures evaluated have included the visual measurement
of mixing, removal of dye from the canal wall, irrigant replacement, irrigant penetration into
secondary/accessory canals, removal of artificially placed debris and histological evaluation
of residual pulp tissue and residual biofilm.
The efficacy and efficiency of such agitation in removing material at the walls of the
canal has been estimated by dyeing the material and measuring the degree of removal.
Endosonic irrigation removed dye from canals quickly compared with syringe irrigation
(23-gauge needle); the latter was effective in removing all the dye only when the canal
was flared and enlarged to an apical size 40 (Teplitsky et al 1987). Irrigant replacement was,
however, found to be equally and highly effective regardless of whether syringe (27-gauge
needle), sonic (Micromega 1500) or ultrasonic (Electro-magnetic device, Cavi-Endo) irrigation
was used in canals of size 30 or 35 (Kahn et al 1995). As expected, irrigant replacement in
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simulated canals within resin blocks could be improved by increasing the number of ultrasonic
file activation cycles (Druttman and Stock 1989).
An electrochemical approach has also been used to measure the extent of irrigant
penetration into secondary canals (Nanzer et al 1989). The study model used transparent
acrylic cells with a main cylindrical canal and secondary horizontal or inclined canals
branching off at various levels. Electrolyte was introduced into the canal and agitated by
an ultrasonically activated file. The transfer of ions by diffusion and convection due to
ultrasonic agitation from the bulk of electrolyte to the electrodes embedded in the secondary
canals was measured. Preliminary data showed that the propagation of ultrasonic waves into
lateral canals was greatly hindered by the inclination of the lateral canals, and irrigation apical
to the file extremity was poor as no convection was induced in the vertical direction (Nanzer
et al 1989). This study provides an example of laboratory simulation that requires extrapolation
of the results to a clinical scenario. An actual study evaluating similar parameters but using the
outcome measure of dissolution of bovine pulp tissue placed into simulated accessory canals
resulted in contradictory findings; ultrasonic agitation of the sodium hypochlorite solution
in the main canal was effective in dissolving the pulp tissue regardless of the position and
angulation of the accessory canals (Al-Jadaa et al 2009). The contradictory findings may
only be reconciled through an understanding of the physical processes inherent in ultrasonic
agitation of the fluid in situ.
Recently, extracted tooth and simulated plastic root canal models were designed by Wu
and Wesselink’s group to specifically investigate the factors influencing the efficacy of debris
removal using ultrasonic irrigation (Lee et al 2004a, 2004b, van der Sluis et al 2005a, 2005b,
2006). Removal of artificially created dentine debris placed in simulated un-instrumented
extensions and irregularities in straight, wide root canals was found to be significantly more
effective with ultrasonic irrigation than with dynamic syringe irrigation (27-gauge needle at
2 mm short of the apical foramen) using 50 mL of 2% sodium hypochlorite solution (Lee
et al 2004a). Canal taper (van der Sluis et al 2005b) and mode of irrigant delivery (continuous
delivery through the ultrasonic device or intermittent delivery through the hand syringe) (van
der Sluis et al 2006) were found to have no significant influence on the efficacy of artificial
debris removal. However, the volume of irrigant used was different rendering any direct
comparison of the two modes of delivery inconclusive. In contrast, Lee et al (2004b) found
that ultrasonic irrigation removal of artificially placed dentine debris from simulated resin
canals (apical size 20) was significantly improved with a larger canal taper. The precise
reasons for the contradictory findings are not clear but may be attributed to the difficulty of
controlling the size and shape of canals in real teeth, which have enormously variable canal
anatomy. The efficacy of debris removal from simulated resin canals was not influenced by
the surface configuration (smooth or fluted) of the ultrasonically activated k-file (van der Sluis
et al 2005a), in agreement with a similar ex vivo study (Munley and Goodell 2007).
One group (Meyers, Beck and Reader) has perfected the use of histological evaluation
of extracted mesial roots of mandibular teeth to evaluate the role of irrigation regimens in
removal of residual pulp tissue and dentine debris from the apical parts of canal systems
and their isthmuses. A series of studies have demonstrated the efficacy of various ultrasonic
activation regimens (Goodman et al 1985, Lev et al 1987, Hiadet et al 1989, Archer et al
1992, Gutarts et al 2005, Burleson et al 2007).
The efficacy of removal of root canal bacteria using ultrasonic agitation has been compared
with syringe irrigation in an in vitro study (Briseno et al 1992), where it was significantly more
effective, and in a randomized controlled clinical trial (Carver et al 2007), where in contrast,
it was significantly less effective. As previously explained, the contradictory findings could
be attributed to the difficulty in controlling the many different variables inherent in the two
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different study types. The innate differences may be enumerated as follows: the nature of root
canal infection (artificial versus real); the concentration of the sodium hypochlorite solution
(1–2% versus 6%); the type of ultrasonic instrument (25 k-file versus 25-gauge needle);
the duration of ultrasonic activation (20 versus 60 s) and the type of ultrasonic machine
(electro-magnetically driven Cavi-Endo unit versus piezo-electrically driven Mini-Endo unit).
A further study using the same outcome measure but different interventions revealed no
significant difference in the efficacy of bacterial elimination when comparing ultrasonic with
manual file agitation of sodium hypochlorite irrigant (Siqueira et al 1997).
5. Application of physical principles to clinical procedures
Clinical trials and laboratory (ex vivo and in vitro) experiments are both important and
complementary in providing evidence for the development of good clinical practice. However,
bridging the gap between the relatively uncontrolled clinical scenario (with the possibility of
unmeasured factors) and the definitively controlled in vitro experiments may prove a challenge
as demonstrated above. The gap between these study types may be bridged by a further class
of experiments that reveal the underlying physical processes; in the case of the problem under
discussion, this means the nature of the flow field within the root canal system. These often
require further abstraction of the clinical setting and the isolation of the physical processes that
dominate the flow field. This allows a general model of these processes to be built up and then
applied to particular circumstances. Techniques for this approach include use of (a) scaled
laboratory experiments; (b) computational fluid dynamics (CFD) and (c) analytical models
or scaling analysis. The behaviour predicted by such models may be validated in controlled
in vitro and ex vivo experiments. Such validated outcomes should allow extrapolation to
realistic, clinical situations and prediction of the likely outcome of different interventions. In
addition, they should also allow interpretation of the outcomes of clinical and ex vivo trials.
There is a large body of well-tested techniques for modelling flow situations to both
understand flows and refine experiments. In particular, the use of scale models to measure
complex flow fields round engineering objects is very well developed (e.g. White 1999,
Douglas 1969). The value of the use of scaled models is that measurements become practically
possible. The models are easier to construct to specification and instrument, and allow the
flow field to be visualized and measured. Model testing relies on dynamic similarity, which
allows the scale-up of the experiments from the size of the tooth to a larger size. The process
requires not only the physical form of the root canal to be scaled, but also the non-dimensional
groups controlling the behaviour to be identical in the scaled-up experiment.
A particularly important non-dimensional number is the Reynolds number,
Re =
ρU L
,
μ
(1)
where ρ is the density of a fluid, μ its viscosity, U its velocity and L is a characteristic length
scale. It expresses the ratio of inertial forces to viscous forces. So, flows with low Reynolds
numbers are dominated by viscous forces and are laminar in nature, whilst those with high
Reynolds numbers, by the effects of mass are turbulent in nature. In an experimental model
of a root canal, it is possible to preserve the right balance between inertial and viscous forces
when the Reynolds numbers of the irrigant in the root canal and the scaled-up experimental
tube are matched; the increase in scale results increases L, and this can be compensated for by
ensuring that the fluid in the experiment has a higher viscosity compared with the root canal
irrigant or the average flow speed, U, is reduced.
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Mixing, in a broad sense, describes the spreading of an initial patch of material over a
larger area. This can also be broadly characterized by the dimensionless group, the Peclet
number, defined by
UL
Pe =
,
(2)
DM
where DM ∼ 10−8 m2 s−1 is the molecular diffusivity of sodium hypochlorite, L is a
characteristic length scale and U is a characteristic velocity scale. This is the ratio of
mechanical mixing of newly introduced fluid (i.e. it is physically mixed with the existing
fluid by the flow field) to molecular diffusion; so, for high Pe, mixing is mediated through the
displacement of the irrigant by stretching and folding, which leads to the spread of the fresh
irrigant over a greater volume, while for low Pe(<1), diffusion is important and the effect of
advection weak.
In many applications, the mixing of fluids is achieved by generating high Reynolds
numbers so that turbulence is produced, and this generates a flow field that promotes mixing
and achieves high Peclet numbers. At low Re and high Pe, turbulence is not generated, and
material distant from the boundaries of the space is passively advected by the streamlines. This
type of agitation is reversible such that a blob of dyed liquid is stretched out when the fluid is
moved, but if the fluid movement is reversed, the blob will be retrieved; the fluid is therefore
ineffectively mixed (a striking demonstration of this can be seen in the film by Taylor (1967)).
For mixing to be effective in viscously dominated systems, the fluid flow needs to be unsteady,
so that the fluid tracks at different times look different and cross each other (Sturman et al
2006). This can be achieved by the performance of a series of transformations on the bolus
so that it is drawn into a filament, distorted, folded or cut (e.g., the ‘baker’s transformation’).
Several of these operations repeated in succession should result in a well-mixed system. Such
mixing is irreversible and known as chaotic mixing because the path of any portion of fluid
within a mixing region is sensitive to its initial position. It is possible for unsteady flows to
have some regions that are well mixed and others that are not (Sturman et al 2006). Mechanical
R
, may generate an unsteady
methods for the introduction of irrigant, such as the RinsEndo
flow field in the root canal, allowing mixing between the new irrigant and the resident canal
fluid, which is in contrast to the scenario with straightforward injection.
The effect of boundaries is important for mixing, especially for high Pe. Owing to the
action of friction, at a solid boundary the fluid velocity is zero. Because of this, in confined
channels large velocity gradients (or shear) can be generated in a fluid flow. The effect of
shear is to enhance cross-stream diffusion so effectively that the longitudinal dispersivity
is dominated by diffusion. In the limit of large Pe, this results in an effective streamwise
diffusivity, that is DM/Pe, which is much larger than DM. This process is called the Taylor–
Aris dispersion (Stone and Brenner 1999). There are many engineering approaches to mixing
fluids in small cavities so that Re is small, but most rely on designing the walls of the channel to
encourage chaotic mixing (e.g. Liu et al 2000). However, this method has limited application
for root canal systems, so unsteady flow fields have to be generated by other means, either
through the delivery of the irrigant or by its agitation by tools.
5.1. Delivery of irrigant
Figure 6(a) shows a schematic diagram of the injection of irrigant through a needle into a
slowly tapering root canal. The canal radius at the end of the needle is defined as D and UN is
the average liquid speed in the needle:
4Q
,
(3)
UN =
π dI2
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50
0.5
0
20
0.4
Gauge 21
10
0
0.35
Gauge 23
0
15
0.3
40 0
Gauge 25
0.25
0
20
0.2
Gauge 27
60 0
40 0
Gauge 30
0.15
15
0
0
2
0.05
0
80 0
10 00
60 0
50
0.1
40 0
60 0
0
10
0
Internal needle diameter, d N mm
0.45
80 0 00
10
0 00
8010
0
1
2
3
4
5
6
Volume flux, Q ml/min
Typical Q during
Manual injection
Typical Q during
EndoVac injection
Figure 8. Contour plot (generated using MatLabs Contour Command) showing the variation of
Reynolds number ReN (equation (7)) with internal needle diameter dN and volume flux of irrigant
(Q mL s–1).
where Q is the injection rate of the irrigant and dI is the internal diameter of the needle. By
considering the parameters that are available to affect mixing,
P e = f (α, Re),
(4)
where α is the relative size of the needle to the canal hole and geometry of the needle tip,
D − dE
(5)
α=
D
and the needle Reynolds number is
UN dI
ReN =
,
(6)
υ
where υ = μ/ρ is the kinematic viscosity of the fluid, UN is the average velocity of the irrigant
in the needle and dE is the external diameter of the needle. Combining (3) and (4), we have
4Q
.
(7)
ReN =
π dI υ
The variation of ReN with the internal needle diameter and injection flux is shown in figure 8.
For a 30-gauge needle, recommended manual injection rates give an approximate value of
ReN ∼ 150, while for automated injection, this increases to ReN ∼ 1500. For continuous
injection, fresh irrigant largely displaces the irrigant–debris mixture. The physics of mixing
changes with Reynolds numbers. Typically, for ReN ∼ O(100), the mixing is expected to
be relatively weak with the transport tending to be through advection, except in certain
circumstances, as discussed later. When ReN is typically large, the flow at the exit of the
needle is likely to be unsteady and may even be on the verge of being turbulent, though the
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flow within the needle and that exhausting up the canal is likely to be steady and laminar.
The effect of additives to the irrigant is usually to increase its viscosity and so reduce its
Reynolds number. Surfactants have also been added to the irrigant and have been found to
improve the efficacy of simulated-biofilm removal (Merivale et al 2009). Since surfactants
tend to affect the interface between air and irrigant, it is more likely that any positive effect
on biofilm removal was either through their chemical action on the biofilm or through the
presence of thickeners, which increase irrigant viscosity.
A well-designed needle would perform several functions, including (a) ensure that the
irrigant penetrates as far as possible into the root canal system; (b) exert large shear stresses on
the canal walls, to remove adherent smear layer, debris or biofilm and (c) efficiently flush out
debris from the canal space. Physical, life-sized models of root canals can be made to examine
the flow field generated during the injection process, but it can be difficult to manufacture them
accurately and take measurements of the flow field. These difficulties can be avoided through
the use of scaled-up models. Some illustrative examples are presented here for 20 mm holes
drilled in polycarbonate to represent the canal where scaled-up needles (dI = 8 mm, dE =
12 mm) are used for delivery of water as the working irrigant into them. The injection volume
flow rate through the needle was controlled to obtain Re = 265. Further insight was obtained
from computational fluid dynamics (CFD) using conservation equations for a Newtonian fluid.
5.1.1. Irrigant penetration. A common feature of all these flows is the presence of a
stagnation plane, a distance dS from the needle tip, beyond which fresh irrigant will not
penetrate after injection. Figure 8 shows the effect of injecting fresh irrigant into a simulated
root canal at Q = 0.02 mL s−1 using a gauge 30 ISO needle.
The presence of the stagnation plane is anticipated even for extremely slow flows
(ReN ∼ 1) and is an example of Moffatt’s corner vortices (Moffatt 1964). Blake (1979)
examined the forcing of a Stokes flow in a circular cylinder and showed the presence of a
series of viscous toroidal eddies rotating in alternate directions, with a stagnation plane lying
at a distance dS /D ∼ 0.5 from the forcing. The velocity decreases so quickly with distance
from the end of the needle that the series of re-circulating vortices usually cannot be identified
experimentally. This anticipated result was reflected in a study measuring the efficacy of
replacing a radio-opaque (Hy-Paque—50%) solution in root canals of human extracted teeth
(Ram 1977). Teeth filled with a radio-opaque solution were irrigated once with 5 mL of a
saline solution through a syringe with a 25-gauge needle placed loosely within the coronal
third of the canals. It was found that the irrigation left the apical half of most canals (diameter
0.25 mm) undisturbed, although increasing irrigation time and pressure caused an additional
2 mm of the radio-opaque solution to be replaced. In contrast, the radio-opaque solution in
wider canals of 0.4 and 0.6 mm diameter was mostly completely replaced.
Figure 9(a) shows a photograph of the four scaled-up needle types used to simulate the
flow in a scaled-up model of the root canal. The experimental study consisted of a vertical
needle located in a cylindrical root canal. Two projected views were taken as a function of
time. The intensity of each image in figure 9(b) represents the integrated effect of dye on a
diffuse light source behind the transparent root canal. The stagnation plane beneath the needle
end is evident where dS /D = 6, 4, 3.5 and 2.3 (for i, ii, iii and iv, respectively). In order
of increasing dS /D (for fixed Q), these physical analogue experiments indicate that in terms
of irrigant penetration, the flat open-ended (FOE) needle is the best, followed by the BOE,
SCOE and CE-SSO needle designs. The needle sides for the BOE and SCOE designs have a
tendency to reduce the momentum of the flow along the canal and reduce dS /D as compared
to the flat open-ended (FOE) needle. The CE-SSO needle generates the shortest penetration
distance beyond the needle end, though these differences are not considerable. Since neither
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(a)
(i)
(iii)
(ii)
(iv)
(b)
1
2
3
4
5
6
x/D
decreasing
dS / D
Figure 9. (a) Needle types used in a scaled laboratory experiment and (b) experimental observations
of the injection of dye into a model root canal. The colour corresponds to the attenuation of the
image due to the presence of dye. Two perpendicular projected views are shown. The white
horizontal line shows the position of the stagnation plane for (i) flat open-ended (FOE) needle,
(ii) bevelled open-ended (BOE) needle, (iii) side-cut open-ended (SCOE) needle and (iv) closedend single side-opening (CE-SSO) needle.
the flat open-ended or BOE needles are contemporarily recommended for use in root canal
treatment, the SCOE needle gives the best performance in terms of dS /D. Reducing the length
of the cut would have a tendency to increase dS /D, as well as placing the side opening much
closer to the end of the needle.
Investigation of fluid flow using the CFD approach has been recently applied to study root
canal irrigation. Boutsioukis et al (2009) examined the flow generated by an ISO 30-gauge
needle penetrating a canal. The geometry of the needle used was taken from measurements
of a needle type used in practice. They considered flow rates from Q = 0.02– 0.79 mL s−1,
though the higher flow rates are rarely used in clinical practice. Their results also demonstrate
the presence of a stagnation plane beyond the end of the needle, but in their simulations this
distance was dS /D ∼ 1–1.2 (case A–C, in Boutsioukis et al 2009 notation). As a demonstration
of this investigation method, figure 10 shows the simulated flow by the authors through a flat
open-ended needle with the same geometry as the laboratory experiments for ReN = 100 and
200. The numerical calculations were undertaken using CFX 5.0 (CFX Ltd, UK) at a resolution
of 1 million nodes to calculate the steady axisymmetric flow due to needle injection into a
cylindrical cavity. The geometry of the needle and canal were the same as the experimental
study shown in figure 9. The stagnation plane is indicated as dS /D = 3.5 and 6 for this range,
which is comparable to the experimental measurements. This distance is much larger than
dS /D ∼ 1 − 1.2 for side hole injection as calculated by Boutsioukis et al (2009), though their
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Topical Review
Re N =100
Re N =200
1
2
Position of
stagnation 3
plane
4
5
6
7
Position of
stagnation
plane
x/D
Figure 10. Isocontours of the fluid speed from a CFD calculation of the flow in a needle and root
canal. The needle geometry is a straight needle type with flat open end (FOE). The position of the
stagnation plane is shown for ReN = 100 and 200.
needle was more loosely fitted in the canal (dE/D = 0.49 rather than 0.60) and the exit hole of
their needle was at a considerable distance from the needle tip.
5.1.2. Wall shear stress. Clinically, it is likely that the biofilm and smear layer are removed
both by the chemical action and physical action of shear stress on the canal wall generated
by fluid flow during irrigation. Wall shear stress is a difficult parameter to measure directly,
but will depend on the flow velocity gradient at the wall. The order of magnitude of the shear
stress on the canal wall exerted by the flow along the canal can be estimated using lubrication
analysis. The shear stress for a Newtonian fluid is defined by
τ =μ
∂u
,
∂y
(8)
where the differential is evaluated on the root canal wall. The shear stress scales as
τ ∼μ
Q
,
π D(D − dE )2
(9)
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and is estimated to lie in the range ∼0.001–0.01 Pa for a flow rate of 0.02–0.2 mL s−1. This
indicates that the shear stress along the canal wall is quite small and is consistent with the
clinical experience that fluid delivery by injection does not typically lead to biofilm or smear
layer removal. It is possible to increase the shear stress by concentrating the flow against the
wall, and generating high local velocity gradients; so, out of the four needle designs shown in
figure 9, the side opening needle has been found to be experimentally effective in removing
biofilm in the area adjacent to the side hole (Huang et al 2008). Designs that use multiple
small holes (Goldman et al 1976) rather than a single large opening can generate faster flow
through these nozzles and will tend to increase the shear stress on the canal wall and improve
biofilm and smear layer removal; however, the addition of side ports will also dramatically
reduce dS /D (figure 9(b, iv)).
5.1.3. Irrigation replenishment. It is important that a large portion of the body of irrigant
within the root canal system is replenished. It is therefore desirable to transport the irrigant
from the coronal and central regions of the canal to its perimeter and the apical regions. Fluid
moves relatively slowly in regions adjacent to the rigid walls, where the non-slip condition is
applied, and so though debris is removed, fresh irrigant would only reach the walls through
diffusion after a finite period of time. Even if the flow is inertially dominated and unsteady
near the end of the needle, it will tend to be laminar along the canal wall. Assuming the
flow is approximately parabolic between the needle and a cylindrical canal wall, to replenish
a fraction β = 99% of the irrigant requires the layer of resident irrigant to have a thickness
δ ∼ (1 − β)(D − dE ). This requires a volume
Qt =
Lπ D(D − dE )
Lπ D(D − dE )2
≈
δ
1−β
(10)
of irrigant to be injected. For low flow rates, this volume is independent of the rate of injection
and is estimated to be 100 times the root canal volume or about 1 mL.
5.2. Agitation of irrigant
Analysis of the fluid mechanics shows that fresh irrigant injected into the root largely displaces
resident irrigant within a displacement dS below the end of the point of injection. Mixing of
new irrigant and the liquid within a root canal can be achieved through diffusion, which takes
a long time, and advection, which is confined only to the upper reaches of the canal if only
injection is relied upon. Further mixing is encouraged either manually, through the application,
or automatically using sonic and ultrasonic devices, but there has been little research on the
mixing that occurs due to agitation in the dental field.
5.2.1. Manual agitation. Agitation by push-pull reciprocating movement of an instrument
may be undertaken using hand files (Cecic et al 1984), irrigation needles (Druttman & Stock
1989) or ‘well-fitting’ tapered gutta-percha points (Huang et al 2008). The key aspect is that
the instrument used must be tightly fitted when pushed into the hole and close to the end of
the canal (Yu 2007). If the end of the canal was not close to the tip of the moving instrument
or the instrument was not tightly fitted, then the instrument would merely pull liquid up and
down in a reversible manner; however, when the instrument tightly fits, it forces liquid to be
displaced down the tube where, if it is unable to be extruded through the canal terminus due to
tissue pressure, it moves sideways and upwards through the gap between the instrument and
the canal wall. It is the latter motion that generates the mixing.
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This process for causing mixing can be illustrated by considering the use of a gutta-percha
cone to agitate the irrigant in the canal, as shown in figure 7(b). Using this form of a circular
cone, it is possible to explain some of the physical processes when it is reciprocated along the
length of the root canal. The vertical movement of the conical agitator (of angle α) generates
a mean flow:
(x − lN )2 UA
(11)
U=
lN (2x − lN )
a distance x > lN from the end of the root canal, and U = 0 for x < lN , below the end of the
agitator. The shear at the distance x from the end of root canal scales as
U
(12)
(x − lN ) tan α
and is extremely large along the entire fluid column between the cone and the root canal wall.
The Peclet number in this situation is given by
UA (x − lN )3 tan α
.
(13)
lN (2x − lN )DI
This may have a high value, but this is deceptive because in the absence of diffusion, the
discharged irrigant would be merely moved up and down the root canal. However, the high
shear accentuates the effects of diffusion by giving rise to the Taylor–Aris dispersion. The
parabolic shear profile causes an enhancement of mean streamwise diffusion with the effective
Peclet number becoming
Pe ∼
Pe
(14)
1 + P e2 /210
(from Stone and Brenner (1999)).
The value of this effective Peclet number (13) is quite small, showing that the role of the
cone is to enhance diffusion and therefore mixing between the fresh and resident irrigants.
The second important effect of the generation of high velocity gradients by the confined
channel between the cone and the canal wall is the high shear stresses that result. The shear
stress on the wall caused by the cone scales as
P eE =
τ∼
μ(x − lN ) UA
μUA
∼
.
lN (2x − lN ) tan α
2lN tan α
(15)
For typical values of UA ∼ 0.02 m s−1, lN ∼ 10 μm, α ∼ 0.04, the shear stress on the root canal
wall is τ ∼ 20 Pa. This is more than three orders of magnitude greater than that generated by
fluid injection and is therefore much more likely to be effective at removing the biofilm from
the canal wall.
5.2.2. Sonic and ultrasonic agitation. Sonic and ultrasonic instruments are used to ‘activate’
the irrigant in situ in the canal. The numerous studies on ultrasonic irrigation have produced
conflicting data (van der Sluis et al 2007). Two potential physical mechanisms underlying
the effect of ultrasonic debridement of root canal systems have been investigated using a
photometric system, cavitation and acoustic micro-streaming (Ahmad et al 1987a). Cavitation
is the generation of vapour within a fluid owing to a drop in pressure; however, the findings
suggested that transient cavitation was unlikely to play a role in canal cleaning (Ahmad et al
1987a). Nevertheless, Shrestha et al (2009) explored the possibility of utilizing such collapsing
cavitation bubbles generated by high-intensity focused ultrasound to deliver antibacterial
nanoparticles into dentinal tubules.
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Figure 11. Ultrasonic agitation of a file at 30 000 Hz. The streaming flow set up by the motion
of the file is visualized by placing the file just below a free surface on which white tracer particles
are suspended. The streak patterns are seen through the long exposure time of the camera. It is
uncertain, what sort of streaming patterns might develop in the constrained environment of the
apical part of root canal anatomy.
Acoustic streaming is the bulk movement of a liquid, generated when pressure waves are
projected through it. It can have many origins, but the main component is often generated by
the dissipation of acoustic energy owing to the viscosity of the fluid, particularly close to solid
boundaries (Lighthill 1978). Ahmad et al (1987a) proposed that acoustic micro-streaming is
potentially the main mechanism involved in effecting canal debridement, and it is influenced
by the size of the file and the ultrasonic power output (Ahmad et al 1987b). The findings
of Ahmad et al (1987a, 1987b) were based on the nature of file oscillation in air or when
submerged in large volumes of fluids and there is an absence of insight into the nature of fluid
movement generated by acoustic instruments within the confined root canal system.
The use of a sonic or ultrasonic agitator has a number of different fluid mechanical aspects.
First, the agitation of an instrument (perpendicular to its axis) generates a streaming motion
with a thin boundary layer whose thickness scales as
δ ∼ (ν/f )1/2 ,
(16)
where f is the frequency of vibration of the instrument (Batchelor 1967, p 354). This gives a
boundary layer thickness of 30 to 5 μm for f = 1000 to 30 kHz. The thin boundary layer and
the streaming motion combine to generate a large shear stress on the surface of the instrument
and root canal wall, which scales as
τ∼
μf ε2
,
dA δ
(17)
where ε is the displacement of the agitator and dA is the diameter of the agitator (see
equation (2) from van der Sluis et al 2007). For typical values, this gives τ ∼ O(10) Pa,
indicating that ultrasonic agitation generates a significant shear stress on the walls of the root
canal. This is supported by the clinical observation that ultrasonic agitation of the irrigant may
be effective in helping dislodge a simulated biofilm or a smear layer. The dissipation within
the flow caused by the movement of the instrument generates heat, which has been observed
to locally increase the fluid temperature from 37 to 45 ◦ C (van der Sluis et al 2007). This
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increase in temperature may result in increased effectiveness of the sodium hypochlorite in
dissolving organic matter.
The ultrasonic agitation of the instrument also generates a large-scale streaming pattern,
as shown in figure 11. Since the dissipation caused by the ultrasonic agitation of the instrument
decreases with the local radius of the instrument (which is usually tapered), the tip tends to
have the largest displacement and generates the largest shear stress. The streaming flow at
the end of the instrument can penetrate into the region of spent resident irrigant located at the
apical portion of the root canal system and it is this that is likely to play an important role in
debriding the biofilm-infected apical anatomy. If the instrument is constrained and confined
in its oscillation, then it is unlikely that this process would occur.
6. Conclusions
Despite the significant advances in root canal treatment, there remains a lack of clarity about
the mechanisms of irrigant delivery, replenishment, mixing, flushing and wall erosion. This
review attempts to demonstrate how knowledge of the physical processes in fluid movement
within root canal systems may help interpret and better explain the outcomes of ex vivo,
microbiological and clinical studies.
The existence of a stagnation plane beyond which the irrigant cannot pass has been
observed in many clinical studies but never has been so defined. The identification of the
phenomenon opens the possibility of solutions to the problem; for example, the distance
between the stagnation plane and the needle tip can be controlled through needle tip design,
fluid flow rate and the relative size of the needle to the canal hole. Furthermore, the low
Reynolds number within the root canal system helps explain the poor mixing of freshly
delivered and debris-laden, spent, resident irrigant. Recognition of the physical processes
allows a better understanding of why the long established clinical procedures of ‘recapitulation’
and ‘patency filing’ help achieve fluid mixing, and therefore prevent canal blockage. This
simple but little regarded step in root canal treatment emerges as the single most important
operator action influencing the outcome of established root canal treatment procedures. Mass
dispersal of irrigant may also be effected by agitation of the irrigant with a well-fitting guttapercha cone or by the medium of a sonically or ultrasonically activated instrument.
Since the key problem in root canal treatment is removal of a surface-adherent bacterial
biofilm, which is not reached with the same degree of ease and access as that of plaque on the
external surface of the tooth by a toothbrush, the action of instruments and fluids is critical.
Yet, direct injection of fluids into the canal is unlikely to effect biofilm removal. However,
the use of side-opening needles may be more effective than end-opening needles because the
Reynolds number is higher at the exit of the side opening of a needle tip and the shear stress
opposite the outlet much higher than for other needle designs. Needles with multiple openings
around the circumference of the needle may provide a means to improve biofilm removal. The
reciprocating action of a well-fitting cone not only enhances mixing of the fluid in the canal
but also increases significantly the shear stress on the canal wall. Sonic or ultrasonic agitation
of the fluid through unconstrained file oscillation can generate large shear stresses on the canal
surface and streaming flow inside the root canal. The additional effect of the increased sodium
hypochlorite solution temperature as a function of ultrasonic agitation may potentially also
lead to increased tissue or bacterial dissolution.
This review has highlighted some of the physical processes that may help elucidate root
canal irrigation but much work remains to validate analytical and numerical models of fluid
behaviour, as well as considering the effects of chemical dissolution on viscosity and fluid
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R79
flow. These models may then be extended to the scale of teeth and tested through clinical
trials, allowing more effective treatments to be devised.
Acknowledgments
The authors gratefully acknowledge the valuable assistance provided by Vladimir Taban in
undertaking the laboratory scale experiments and that provided by Alireza Azarbadegan in the
CFD calculations shown in figure 10.
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